Asset Publisher

No. 242 – End of Earth occultation season #12; start of Atmospheric Drag Experiment campaign #7

No. 242 – End of Earth occultation season #12; start of Atmospheric Drag Experiment campaign #7

Report for the period 18 December 2011 to 14 January 2012 This reporting period covers four weeks of Venus Express mission operations, from 18 December 2011 to 14 January 2012. The period covers routine operations at moderate but increasing telemetry downlink data rates, the end of the twelfth Earth occultation season, and the beginning of the seventh Atmospheric Drag Experiment campaign.

Cebreros ground station

Operations with the Cebreros (CEB) ground station were nominal.

There were data gaps in the science packets generated on 13 December for the MAG (magnetometer), and SPICAV (atmospheric spectrometer) and VIRTIS (imaging spectrometer) instruments. The cause is not known. However, a copy of the data is retained on-board the spacecraft until its receipt on the ground is verified. In this case, the data were retained on board until there was time in the schedule to downlink it to the ground again. The data were successfully downlinked and retrieved on 29 December.

Seventh Atmospheric Drag Experiment campaign

The seventh Atmospheric Drag Experiment (ADE) campaign started on 8 January 2012, as the pericentre altitude approached 175 kilometres above the surface of Venus. This is the latest campaign in a series that have been performed regularly since 2008. These campaigns provide measurements that are used to improve models of Venus's upper atmosphere, especially over its north pole.

The ESA ground station at New Norcia (NNO), Australia, and the NASA Deep Space Network (DSN) station in Canberra, Australia, provided tracking at pericentre during this period.

Venus Express's parking orbit is highly eccentric; it takes the spacecraft out to roughly 66 000 kilometres from the planet at its apocentre, and much closer to the planet's surface when the spacecraft is at pericentre.

For the ADE campaign, the entire spacecraft body and the attitude-sensing gyroscopes act as an instrument that detects the drag exerted by the atmosphere. By measuring its integrated effect on the spacecraft's attitude and position in orbit, the atmospheric density can be determined. During the campaign, the spacecraft's pericentre is allowed to decrease beyond its nominal altitude, down to 175 kilometres in this case (see figure) as the spacecraft flew over the northern pole of Venus. During closest approach, the spacecraft's solar panels are tilted to induce a very tiny roll in the spacecraft. This roll is countered by the reaction wheels, and the change in reaction wheel speeds can be used to determine the density of the atmosphere at this extreme upper altitude. The amount of drag experienced by the spacecraft is significantly lower than that which would be experienced during an aerobraking orbit change. The drag is maintained this level, which is known to cause no problems for the spacecraft.

The regular wheel off-loadings (WOL) that are used to release the accumulated spacecraft momentum every day were performed in the middle of the daily Cebreros communications passes. This provided at least one hour of tracking after the WOLs, allowing even minor orbit perturbations from the WOLs to be determined and removed from the orbit data, thus improving the accuracy of the ADE results.

Change in Venus Express's pericentre altitude with time. Credit: ESA

Occultation season #12

The twelfth Earth occultation season ended on 06 January 2012.

During occultation seasons, the geometry places Venus between the spacecraft and Earth for part of each orbit. This alignment allows the use of the spacecraft's Earth communications signal to probe the planet's atmosphere.

Shortly before Venus Express reaches pericentre, it is pointed to Earth and broadcasts its default signal. The signal used for occultation measurements must be far more accurate than required during a communications pass, hence it is generated using a special oscillator. As the spacecraft moves behind the planet, this highly accurate signal passes through the Venusian atmosphere before being blocked by the planet. The refracted signal can be measured and processed to yield details about the atmosphere that cannot be obtained in other ways. These occultation measurements are performed when Venus Express is at its pericentre, as and when the Earth-Venus-spacecraft geometry allows.

The ESA ground station at NNO was used as for occultation measurements, as the station faced Venus when the spacecraft was at pericentre.

The next occultation season beginning 27 March 2012 will be extremely unusual: the season's Earth-Venus-spacecraft geometry will last for about six months, as opposed to the typical two months for most occultation seasons.

Venus Express will fly over the terminator on 10 January, which happens roughly every four months. Contact with the Cebreros ground station will be skipped during this period, allowing the VMC (monitoring camera) to take extended images along the terminator in a thermally benign configuration.

Summary of main activities

The table below shows a chronology of the main spacecraft bus activities in the reporting period:

Main activities during reporting period
MET
(Day)
Date DOY Main Activity

MET = Mission elapsed time; DOY = Day of year; EOC = End of Charge; CEB = Cebreros; NNO = New Norcia; ADE = Atmospheric Drag Experiment; RSI = Radio Science Investigation

2231 18-Dec-2011 352 CEB communication pass. Pre-ADE#7 operation.
2232 19-Dec-2011 353 CEB communication pass. Occultation RSI over NNO around pericentre.
2233 20-Dec-2011 354 CEB communication pass. Occultation RSI over NNO around pericentre.
2234 21-Dec-2011 355 CEB communication pass.
2235 22-Dec-2011 356 CEB communication pass. Occultation RSI over NNO around pericentre.
2236 23-Dec-2011 357 CEB communication pass. Occultation RSI over NNO around pericentre.
2237 24-Dec-2011 358 CEB communication pass.
2238 25-Dec-2011 359 CEB communication pass. Occultation RSI over NNO around pericentre.
2239 26-Dec-2011 360 CEB communication pass. Occultation RSI over NNO around pericentre.
2240 27-Dec-2011 361 CEB communications pass.
2241 28-Dec-2011 362 CEB communication pass. Occultation RSI over NNO around pericentre.
2242 29-Dec-2011 363 CEB communication pass. Occultation RSI over NNO around pericentre.
2243 30-Dec-2011 364 CEB communication pass.
2244 31-Dec-2011 365 CEB communication pass.
2245 01-Jan-2011 01 CEB communication pass.
2246 02-Jan-2011 02 NNO occultation pass. CEB communications pass.
2247 03-Jan-2011 03 NNO occultation pass. CEB communications pass.
2248 04-Jan-2011 04 NNO occultation pass. CEB communications pass.
2249 05-Jan-2011 05 NNO occultation pass. CEB communications pass.
2250 06-Jan-2011 06 NNO occultation pass. CEB communications pass.
2251 07-Jan-2011 07 NNO occultation pass. CEB communications pass.
2252 08-Jan-2011 08 CEB communication pass.
2253 09-Jan-2011 09 ADE#7. Uplink frequency ramping test during NNO pass. CEB communication pass.
2254 10-Jan-2011 10 Seventh ADE. CEB communication pass.
2255 11-Jan-2011 11 Seventh ADE. CEB communication pass.
2256 12-Jan-2011 12 Seventh ADE. CEB communication pass.
2257 13-Jan-2011 13 Seventh ADE. CEB communication pass.
2258 14-Jan-2011 14 Seventh ADE. CEB communication pass.

At the end of the reporting period on 14 January, Venus Express was 181.5 million kilometres from Earth; the one-way signal travel time was 605 seconds. The final oxidiser mass was 29.993 kg and the final fuel mass was 18.639 kg.

Payload activities

The instruments were operated nominally as per the plans of each instrument team.

 

ASPERA The instrument was regularly operated as part of the routine plan.
MAG The instrument was regularly operated as part of the routine plan.
PFS The instrument was not operated.
SPICAV The instrument was regularly operated as part of the routine plan.
VMC The instrument was regularly operated as part of the routine plan.
VeRa The instrument was operated for the occultation passes, at orbit pericentre.
VIRTIS The instrument was regularly operated as part of the routine plan.

Future milestones

  • Orbital correction manoeuvres to raise the pericentre after the Atmospheric Drag Experiment.
  • Start of the twentieth eclipse season on 8 February 2012. As the spacecraft passes into and then out of the shadow of Venus, the SPICAV and VIRTIS spectrometers can measure the changes in solar radiation as it passes through the different atmospheric layers.
  • Quadrature operations: as the Earth-Venus-Sun angle exceeds 90 degrees, changes in antenna usage and restrictive thermal constraints become necessary. Quadrature season starts in March 2012.


---
Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #317 through MOR #320. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.

Last Update: 1 September 2019
9-Mar-2021 11:18 UT

ShortUrl Portlet

Shortcut URL

https://sci.esa.int/s/8DRpeVw

Related Publications

Documentation